if you had a ship with 250 attack and no defense vs a ship with 125 attack and 125 defense, the balanced ship I believe would win, although probably be severely damaged.....because 125 defense is enough to handle the 250 attack, but no defense on the 250 attack ship means that all of the 125 attack gets through, so tech being equal and similar ship types (large vs large), you're better off with balanced ships of both one weapon type and some defense...
The all attack ship won't take any damage if it has the first strike advantage and can take out the defender before it can return fire! This is likely if the attacker also spends just 1 race point at setup to get a 25% luck bonus which means he will roll attacks between 62-250 vs. 0-250, for an average attack of 156 vs. the defender's average defense of 62(0-125)!
Recall also that a weapon type is optimal against two defense types whereas, a defense is only optimal against one weapon type. The attacker not the defender chooses the battle and thus attacks with ships optimized to take out the defender.
Finally, in one-on-one ship battles, which are the norm when clearing planetary defenders, in the case of mutual destruction the attack survives with 1 HP (due to a bug usually is restored to full health).